The major stock market indexes turned in their worst week since July after Caterpillar Inc., one of the world's largest construction equipment makers, soured investors' mood Friday with a discouraging assessment of the U.S. economy.

In a week dominated by mostly negative results from banks facing difficult credit markets and rising mortgage delinquencies, investors appeared surprised that an industrial name was feeling an economic pinch, too.

Reports from Honeywell International Inc. and 3M Co., themselves big industrial names, gave investors little incentive to take chances on the market. In one bright spot, Google Inc. rose after reporting stronger-than-expected profits.

Investor sentiment took another hit when Standard & Poor's downgraded another batch of residential mortgage-backed securities, adding to investor unease about credit quality. The latest reduction follows a similar move earlier in the week and affects more than 1,400 classes.

And oil prices appeared on some investors' list of worries after briefly moving above the psychological barrier of $90 per barrel for the first time.

"I was not surprised there was some correction, given our expectation that earnings growth was going to fall short of expectations," said Alan Gayle, senior investment strategist, director of asset allocation for Trusco Capital Management.

"I think stock analysts were slow to incorporate the impact of the subprime crisis on third-quarter earnings," he added.

The Dow fell 366.94, or 2.64 percent, to 13,522.02. The Dow was down for the fifth straight session and for the week was off 4.05 percent. For the year, the blue chip index is now up 8.5 percent.

For the week, the S&P; 500 fell 3.92 percent and the Nasdaq fell 2.87 percent.

Friday's pullback pales in comparison to what investors had to contend with 20 years ago. On Oct. 19, 1987 - Black Monday - the Dow plunged 23 percent amid concerns about interest rates and slowing economic growth. A decline of similar proportion given the market's current levels would mean a drop of some 3,100 points.

Friday's decline - the third biggest point and percentage drop this year - was the ninth biggest point drop in the Dow since Black Monday.

A drop Friday in the NYSE composite index proved steep enough to trigger trading curbs, which puts restrictions on certain kinds of sell orders and are meant to help stabilize the market. These type of protections were put in place as part of the response to Black Monday.